Process for producing cysteinylglycine-enriched food material and process for producing flavor-enhancing agent

ABSTRACT

Disclosed in this application are a method for producing a food material containing cysteinylglycine at a high content, which comprises the step of (a) maintaining a starting food material containing glutathione in a ratio of 1% by weight or more based on the solid content at a temperature of 50 to 120° C. and a pH of 1 to 7 in the presence of water, or (b) treating the food material with a γ-glutamylpeptide hydrolase at a temperature of 15 to 70° C. and a pH of 3 to 9 in the presence of water, whereby a food material rich in cysteinylglycine is allowed to result, as well as a method for producing a food flavor (or savor) enhancer, which comprises the steps of (a) adding a sugar to cysteinylglycine or a food material containing cysteinylglycine in a ratio of 0.5% by weight or more based on the solid content, and (b) heating the resulting mixture at a temperature of 70 to 180° C. for a period of 10 to 180 minutes in the presence of water, whereby a natural food material containing crysteinylglycine at a high content, and therefore, a food savor enhancer rich in cysteinylglycine, can be easily provided.

TECHNICAL FIELD

[0001] The present invention relates to a food material containingcysteinylglycine at a high content, and to a food flavor enhancer, i.e.,a food savor enhancer. In greater detail, it relates to a method forproducing a food material containing cysteinylglycine at a high content,which comprises the step of (a) maintaining a starting food materialcontaining glutathione in a ratio of 1% by weight or more based on thesolid content at a temperature of 50 to 120° C. and a pH of 1 to 7 inthe presence of water, or (b) treating the food material with aγ-glutamylpeptide hydrolase at a temperature of 15 to 70° C. and a pH of3 to 9 in the presence of water, whereby a food material rich incysteinylglycine is allowed to result, and to a method for producing afood flavor enhancer, which comprises the steps of (a) adding a sugar tocysteinylglycine or a food material containing cysteinylglycine in aratio of 0.5% by weight or more based on the solid content, and (b)heating the resulting mixture at a temperature of 70 to 180° C. for aperiod of 10 to 180 minutes in the presence of water.

[0002] Incidentally, in the present specification, the term“glutathione” includes the oxidized form (in the broad sense), and theterm “cysteinylglycine” also includes the oxidized form (in the broadsense), unless otherwise construed in the context.

BACKGROUND ART

[0003] Recently, the demand for processed foods having a meat-likeflavor has been increased with westernization and diversification of theeating habits in Japan. Particularly, in the production of soups,processed meat products such as ham, sausage and the like, and cookedfoods such as hamburger and the like, there is an increasing demand forgood tasting and inexpensive meat flavor.

[0004] Heretofore, as a means for imparting meat flavor to foods withoutusing meat itself or for the purpose of general improvement of theflavor or savor of foods is used a sulfur-containing compound such asglutathione, cysteine, glutamylcysteine or the like. For example, it hasbeen already known in, e.g., Japanese Patent Application Laid-Open(Kokai) Nos. 66069/'92, 91762/'92 etc., that food materials obtainableby heating glutathione or γ-glutamylcysteine, or a yeast extract or thelike containing glutathione or γ-glutamylcysteine in the presence of asugar, are effective for enhancing food savor.

[0005] In greater detail, the former (Japanese Patent ApplicationLaid-Open (Kokai) No. 66069/'92) describes as follows: “as a result oftheir extensive studies for obtaining more preferable meat flavorseasonings, the present inventors have found that a roast meatflavor-like seasoning having a good quality and excellent stabilitywithout the afore-mentioned unpleasant odor and taste derived fromyeast, can be obtained by adding a sugar and an amino acid, if required,to a yeast extract containing a sulfur-containing compound such asglutathione, cysteine, glutamylcysteine or the like in an amount of 2 to20% by weight (solid content concentration), followed by heating theresulting mixture in the absence of fat” (in the beginning of the “Meansfor solving the Problems” section of the patent document) and, based onsuch findings, they have accomplished “a method for producing aseasoning which comprises steps of adding a sugar and an amino acid, ifrequired, to a yeast extract containing a sulfur-containing compoundsuch as glutathione, cysteine, glutamylcysteine or the like in apredetermined amount (2 to 20% by weight of the extract), and heatingthen the mixture in the absence of fat at 70 to 180° C. for 10 to 180minutes” (in the “Claims” section of the patent document).

[0006] The latter (Japanese Patent Application Laid-Open (Kokai) No.91762/'92) describes as follows: “a variety of conventional meat flavorshave been known, but they are all have a different quality from thearoma of roasted natural meat, and therefore, a flavor imparting agenthaving a closer quality to roasted natural meat flavor has been wanted.”(in the “Problems to be Solved by the Invention” section of the patentdocument); and in the situation, “as a result of studies of a flavor ofheated and browned various amino acids and sugars for the purpose ofsolving these problems, the present inventors have found that a flavorcomposition having a pleasant roasted meat aroma or flavor can beproduced by adding a sugar to γ-glutamylcysteine and dissolving theresulting mixture in water, followed by subjecting the solution to aheat reaction by heating at 70 to 180° C. for 10 to 180 minutes, andthus accomplished the present invention” (in the “Means for Solving theproblems” section of the patent document). Thus, it discloses “a methodfor producing a flavor composition comprising the steps of adding asugar to γ-glutamylcysteine and heating the resulting mixture at 70 to180° C. for 10 to 180 minutes”.

[0007] By either method, however, meat flavor is indeed increased, butproblems of a burnt odor, a chemical odor and the like are pointed out.

[0008] In addition, there are problems that the seasonings produced bythe above methods have an unpleasant, strongly pungent taste owing tocysteine or γ-glutamylcysteine, and the seasonings produced by usingglutathione have an unclear taste.

[0009] On the other hand, cysteinylglycine is known as one ofsulfur-containing compounds. Cysteinylglycine is a dipeptide obtainableby combination of cysteine and glycine through peptide linkage, and isconsidered to be a possible pleasant meat flavor enhancing materialsimilar to cysteine and glutathione. However, as methods for producingcysteinylglycine, indeed, a peptide synthesis has been known, but aproducing method from a natural material has hitherto not been known. Inthe case that cysteinylglycine is to be used for the above purpose,there is a strong demand for a natural food material containing the sameat a high content, but such a natural food material containingcysteinylglycine at a high content has hitherto not been known.

DISCLOSURE OF THE INVENTION

[0010] Under the background of the background art described above, it isan object of the present invention to provide a food material containingcysteinylglycine at a high content, and further a food savor enhancercontaining cysteinylglycine at a high content, having only weak burntodor or smell and chemical odor, exhibiting no above-described problemsin the taste, and enhancing savor such as good meat flavor and the likesuitable for foods and drinks.

[0011] As a result of their extensive and intensive studies forachieving the afore-mentioned object, the present inventors have foundthat glutathione when heated at a specific pH and temperature decomposesinto cycteinylglycine and PCA (pyrrolidonecarboxylic acid) to affordcysteinylglycine in high yields as a whole, and that glutathione whentreated with a γ-glutamylpeptide hydrolase decomposes intocycteinylglycine and glutamic acid to afford cysteinylglycine in highyields as a whole. Furthermore, they have found that the heating ofyeast cells or yeast extract containing glutathione in an amount of 1%by weight or more in a hydrous state or the treating of such yeast cellsor yeast extract with a γ-glutamylpeptide hydrolase affords acysteinylglycine-highly containing yeast extract, i.e., a yeast extractrich in cysteinylglycine, and that a food savor enhancer weak in burntodor and chemical odor and having a pleasant taste quality, can beobtained by adding a sugar to cysteinylglycine or a food materialcontaining cysteinyiglycine in a ratio of 0.5% by weight or more basedon the solid content, dissolving the resulting mixture in water, andthen heating (reacting) the resulting solution at a temperature of 70 to180° C. for 10 to 180 minutes. Based on these findings, the presentinvention has been accomplished.

[0012] Accordingly, the present invention relates to a method forproducing a food material containing cysteinylglycine at a high content,which comprises the step of (a) maintaining a starting food materialcontaining glutathione in a ratio of 1% by weight or more based on thesolid content at a temperature of 50 to 120° C. and a pH of 1 to 7 inthe presence of water, or (b) treating the food material with aγ-glutamylpeptide hydrolase (i.e., allowing a γ-glutamylpeptidehydrolase to act on the food material) at a temperature of 15 to 70° C.and a pH of 3 to 9 in the presence of water, whereby a food materialrich in cysteinylglycine is allowed to result, and to such a methodwherein the said starting food material is a yeast extract or a yeastcell which is a natural food material, as well as to a method forproducing a food savor or flavor enhancer, which comprises the steps of(a) adding a sugar to cysteinylglycine or a food material containingcysteinylglycine in a ratio of 0.5% by weight or more based on the solidcontent, and (b) heating the resulting mixture at a temperature of 70 to180° C. for a period of 10 to 180 minutes in the presence of water.

[0013] Hereinafter will be explained the present invention in greaterdetail.

[0014] The natural food material which is a raw material according tothe method for producing the food material containing cysteinylglycineat a high content of the present invention, is a natural food materialcontaining glutathione at a high content, i.e., in a ratio of 1% byweight or more, preferably 4% by weight or more based on the solidcontent. A natural food material containing glutathione at a content of1% or less is not preferred, because a savor enhancer produced therefromexhibits only unsatisfactory savor enhancing effect when such materialis used in the production of the food savor enhancer of the presentinvention which will be explained below.

[0015] As such natural food materials, there may be mentioned yeastcells and yeast extracts containing glutathione in a ratio of 1% byweight or more, preferably 2% by weight or more, and more preferably 4%by weight or more based on the solid content. Incidentally, such yeastcells containing glutathione at a high content can be obtained bymodifying the method of culturing yeasts. Moreover, such yeast extractscan be prepared by extraction of such yeast cells or self-digestionthereof, or by supplementing with glutathione, yeast extracts whoseglutathione content is less than the above predetermined amount.

[0016] According to the present invention, heating such natural foodmaterial at 50 to 120° C. at a pH range of acidic to neutral, i.e., pH 1to 7 in the presence of water results in decomposition of glutathionewithin 3 to 300 minutes into cysteinylglycine and PCA. The amount ofwater present therein is determined in view of the operationability. Forexample, the amount may be from 1 to 100 parts by weight per 1 part byweight (dry basis) of the natural food material. The pH which is out ofthe above range is not preferred because of a smaller production ofcysteinylglycine. Adjustment of the pH can be, needless to say, effectedby an acid such as hydrochloric acid or the like, or a base such assodium hydroxide or the like, which are acceptable for foods. Regardingthe heating temperature, too low a temperature is not preferred becausedecomposition of glutathione proceeds only slowly, and too high atemperature is also not preferred because the cysteinylglycine onceformed is decreased by side reactions. Regarding the heating time, tooshort a period is not preferred because of incomplete reaction, and toolong a period is also not preferred because of the decrease of thecysteinylglycine once formed by side reactions.

[0017] In addition, a method of treatment with an enzyme may bementioned for producing cysteinylglycine by decomposition ofglutathione. Namely, when a food material containing glutathione istreated with an enzyme capable of hydrolyzing γ-glutamyl group, i.e., anenzyme capable of hydrolyzing γ-glutamyl group is allowed to act on afood material containing glutathione, in the presence of water at a pHof 3 to 9 and a temperature of 15 to 70° C., the glutathione isdecomposed to form cysteinylglycine within 1 to 300 minutes. The amountof water present therein is determined in view of the operationability.For example, the amount may be from 1 to 100 parts by weight per 1 partby weight (dry basis) of the natural food material. The pH andtemperature which are out of the above ranges are not preferred becausethe activity of the γ-glutamylpeptide hydrolase is lowered.

[0018] There exist many γ-glutamylpeptide hydrolase. In particular,industrially useful examples thereof include γ-glutamyltransferase,γ-glutamylcyclotransferase, glutaminase, and the like.

[0019] The cysteinylglycine-highly containing natural food material thusprepared may be put into distribution in suitable forms, for example, asit is, i.e., as a liquid, or as a paste after concentration, as a powderafter drying, as granules, or the like.

[0020] Next, the method for producing a food savor enhancer, or a foodflavor enhancer, of the present invention will be explained.

[0021] For example, the desired savor enhancer can be produced by addinga sugar to cysteinylglycine or a natural food material containingcysteinylglycine in a ratio of 0.5% by weight or more, preferably 1% byweight or more, and more preferably 2% by weight or more based on thesolid content, followed by dissolving the mixture in water, and heating(for reaction) the resulting solution at a temperature of 70 to 180° C.,whereby the desired savor enhancer results within 10 to 180 minutes. Bythe way, the molecular weight of cysteinylglycine is almost a half ofthe molecular weight of glutathione.

[0022] The food material which is a raw material in such method forproducing a food savor enhancer, is a food material containingcysteinylglycine at such a high content, i.e., in a ratio of 0.5% byweight or more, preferably 1% by weight or more, and more preferably 2%by weight or more based on the solid content. The food material,needless to say, includes the food material produced by the method forproducing a food material containing cysteinylglycine at a high contentof the present invention explained in the above. The material containingcysteinylglycine in an amount of 0.5% by weight or less is industriallyimpractical.

[0023] As such natural food materials, there may be mentioned yeastcells and yeast extracts containing cysteinylglycine in a ratio of 0.5%by weight or more based on the solid content. Incidentally, such yeastcells containing cysteinylglycine at a high content can be obtained bymodifying the method of culturing yeasts. Moreover, such yeast extractscan be prepared by extraction of such yeast cells or self-digestionthereof, or by supplementing with cysteinylglycine, yeast extracts whosecysteinylglycine content is less than the above predetermined amount.This is the same as has been described above, in connection with themethod for producing the food material containing cysteinylglycine at ahigh content of the present invention.

[0024] Examples of the sugar to be used according to the presentinvention include monosaccharides such as xylose, glucose, fructose,ribose, and the like, and disaccharides such as sucrose, maltose, andthe like.

[0025] The amount of sugar to be added (amount to be used) is determinedas a ratio to the cysteinylglycine, and is from 5 to 400 parts by weightthereto. The concentration of a mixture of cysteinylglycine and sugardissolved in water is from 2 to 50%. At that time, a pleasant meatflavor can be also produced by adding optionally an amino acid such assodium glutamate or a protein hydrolyzate, or a yeast extract to theheating reaction system. Needless to say, other sulfur-containing aminoacids such as cysteine, glutathione and the like may be present in thereaction system.

[0026] The food savor enhancer thus prepared may be put intodistribution in suitable forms, for example, as it is, i.e., as aliquid, or as a paste after concentration, as a powder after drying, asgranules, or the like, as in the case of the food material containingcysteinylglycine at a high content.

BRIEF DESCRIPTION OF DRAWINGS

[0027]FIG. 1 shows the change with the passage of time in the amount ofcysteinylglycine formed resulting from decomposition of reduced-formglutathione by the action of glutaminase at pH 7 (Test Example 1).

[0028]FIG. 2 shows the change with the passage of time in the amount ofcysteinylglycine formed resulting from decomposition of reduced-formglutathione present in a highly glutathione-containing yeast extract bythe action of glutaminase at pH 7 (Example 1).

[0029]FIG. 3 shows the change with the passage of time in the amount ofcysteinylglycine formed resulting from decomposition of reduced-formglutathione contained in a highly glutathione-containing yeast extractby heating (Example 2).

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] Hereinbelow, the present invention in more detail by reference toTest Example and Examples.

TEST EXAMPLE 1

[0031] The amount (increase and decrease, i.e., change with the passageof time) of the cysteinylglycine formed was examined when an aqueoussolution of reduced-form glutathione having a concentration of 50 mmolat pH 7.0 was added with a glutaminase preparation “Glutaminase Daiwa”(specific activity: 3.0 mM/min/mg) manufactured by Daiwa Kasei K. K. inan amount of 1 mg/ml thereof and treated therewith at 37° C. for 3hours, whereby the glutaminase was allowed to act on the glutathione.The results will be shown later in FIG. 1. In the figure, “GSH”represents the reduced-form glutathione, and “Total Cys-Gly” representsthe cysteinylglycine.

[0032] As can be understood from FIG. 1, cysteinylglycine is obtainedfrom glutathione in high yields by the action of glutaminase.

EXAMPLE 1

[0033] Water was added to a yeast extract powder whose glutathionecontent was 8%, and the pH of the resulting mixture was adjusted to 7with sodium hydroxide to prepare an aqueous solution of 10%concentration. To the solution was added the same glutaminasepreparation as that used in Test Example 1 in an amount of 1 mg/ml ofthe solution. Then, the resulting mixture was incubated at 37° C. for120 minutes, followed by freeze-drying, whereby a yeast extract powderhaving a 4.5% cysteinylglycine content was obtained.

[0034] The increase and decrease of the cysteinylglycine with thepassage of time was measured during the incubation. The results will beshown later in FIG. 2. In the figure, the vertical axis represents thecontents of the glutathione and the cysteinylglycine calculated based onthe solid content, respectively.

[0035] Incidentally, when the yeast extract powder was used for theseasoning production, a seasoning exhibiting a pleasant roast meatflavor was obtained.

EXAMPLE 2

[0036] Water was added to a yeast extract powder whose glutathionecontent was 8%, and the pH of the resulting mixture was adjusted to 5with hydrochloric acid to prepare an aqueous solution of 2%concentration. The solution was heated at 98° C. for 180 minutes andthen freeze-dried to obtain a yeast extract powder having a 2.0%cysteinylglycine content.

[0037] During the incubation, measurement was carried out as in Example1, and the results will be shown later in FIG. 3.

[0038] Incidentally, when the yeast extract powder was used for theseasoning production, a seasoning having a pleasant roast meat flavorwas obtained.

EXAMPLE 3

[0039] In 100 ml of water was dissolved 5 g of xylose. Into the solutionwas dissolved 1 g of one of cysteine, glutathione, and cysteinylglycine.The three, thus resulting solutions were heat-treated at 95° C. for 2hours. Sensory evaluation on the flavor of each resulting solution wascarried out by 15 panel persons. The results will be shown below inTable 1. TABLE 1 Cysteine Glutathione Cysteinylglycine Strength of 4.64.4 4.5 roast aroma Preference of 3.2 4.1 4.9 roast aroma Strength of4.3 4.1 4.0 total aroma Preference of 3.4 4.3 4.9 total aroma Strengthof 4.9 4.1 1.0 chemical odor Strength of 4.9 4.9 1.5 pungent odor Score5: very strong or very pleasant 4: strong or pleasant 3: slightly strongor slightly pleasant 2: slightly weak or slightly unpleasant 1: weak orunpleasant Average value at 5-step rating by 15 panel persons

EXAMPLE 4

[0040] In 100 ml of water was dissolved 5 g of xylose. To the solutionwas added a yeast extract containing glutathione at a high content(content of glutathione being 8%; referred to herein as Yeast extract(a)) or another yeast extract obtained by treating the yeast extractcontaining glutathione at a high content with 1 mg/ml of the glutaminasepreparation at 37° C. for 2 hours (no glutathione was detected; contentof cysteinylglycine being 4.5%; referred to herein as Yeast extract(b)). The two, thus obtained solutions were heat-treated at 95° C. for 2hours. Sensory evaluation on the flavor of each resulting solution wascarried out by 15 panel persons, as in Example 3. The results will beshown below in Table 2. TABLE 2 Yeast extract (a) Yeast extract (b)Strength of 4.4 4.4 roast aroma Preference of 4.1 4.8 roast aromaStrength of 4.1 4.0 total aroma Preference of 4.2 4.9 total aromaStrength of 3.9 1.0 chemical odor Strength of 4.9 1.5 pungent odor Score5: very strong or very pleasant 4: strong or pleasant 3: slightly strongor slightly pleasant 2: slightly weak or slightly unpleasant 1: weak orunpleasant Average value at 5-step rating by 15 panel persons

EXAMPLE 5

[0041] Water was added to 15 g of a hydrolyzed vegetable protein (HVP)and 1.5 g of maltose to prepare an aqueous solution in an amount of 50ml in total. Thereto was added 0.5 g of one of cysteine, glutathione,and cysteinylglycine. The three, thus resulting solutions were heated at95° C. for 2 hours, followed by cooling on standing to room temperature,and then the pH thereof was adjusted to 5.5. Thereafter, sensoryevaluation on the aroma, savor and taste of each resulting solution wascarried out by 15 panel persons. The results will be shown below inTable 3. TABLE 3 Cysteine Glutathione Cysteinylglycine Strength of 3.94.1 4.1 roast aroma Preference of 3.4 4.0 4.7 roast aroma Strength of3.8 3.7 3.6 total aroma Preference of 3.2 3.9 4.3 total aroma Strengthof 4.9 3.7 1.0 chemical odor Strength of 4.9 4.9 1.7 pungent odorStrength of 3.4 4.3 4.1 meat flavor Preference of 3.5 4.1 4.4 meatflavor Strength of 4.1 3.6 3.9 total flavor Preference of 3.3 3.5 4.6total flavor Pungent taste 4.9 4.6 1.3 Unclearness of 1.5 5.0 1.3 tasteComprehensive 3.6 4.2 4.8 evaluation Score 5: very strong or verypleasant 4: strong or pleasant 3: slightly strong or slightly pleasant2: slightly weak or slightly unpleasant 1: weak or unpleasant Averagevalue at 5-step rating by 15 panel persons

EXAMPLE 6

[0042] In 100 ml of water was dissolved 5 g of xylose, and then 1 g ofcysteinylglycine was dissolved. The resulting solution was heat-treatedat 95° C. for 2 hours. The thus treated solution was concentrated andfreeze-dried. The resulting dried product (inventive one) was added to acommercially available beef consomme soup in an amount of 0.05% relativeto the solid content. Sensory evaluation by the paired comparison testwas carried out by 15 panel persons, using a corresponding xylose-freeproduct as the control. The results will be shown below in Table 4.TABLE 4 Inventive Control (persons) (persons) Strength of meat flavor 150 Preference of meat flavor 15 0 Strength of total flavor 14 1Preference of total flavor 14 1 Comprehensive evaluation 14 1

EXAMPLE 7

[0043] In 100 ml of water was dissolved 5 g of maltose. To the resultingsolution was added a yeast extract containing glutathione at a highcontent (content of glutathione being 8%; referred to herein as Yeastextract (a)) or another yeast extract obtained by treating the yeastextract containing glutathione at a high content with 1 mg/ml of theglutaminase preparation (the same one as used in Test Example 1) at 37°C. for 2 hours (no glutathione was detected; content of cysteinylglycinebeing 4.5%; referred to herein as Yeast extract (b)). The two, thusresulting solutions were heat-treated at 95° C. for 2 hours. Eachsolution was concentrated and spray-dried to form powder. Thereafter,sensory evaluation on these dried products were carried out inconnection with each system of (a) a commercially available beefconsomme soup, (b) a curry sauce, and (c) a Japanese consomme soup. Theresults will be shown below in Table 5. Each dried product was added inamounts shown in the table based on the solid content, and sensoryevaluation by the paired comparison test of Yeast extract (b) of thepresent invention was carried out by 15 panel persons as in Example 6,using Yeast extract (a) as the control. The results will be shown belowin Table 5. TABLE 5 (0.05% by weight added) Inventive Control (a) Beefconsomme soup, (persons) (persons) Strength of meat flavor  8 7Preference of meat flavor 12 3 Strength of total flavor  6 9 Preferenceof total flavor 12 3 Preference of taste 14 1 Strength of taste 15 0Comprehensive evaluation 13 2 (0.1% by weight added) Inventive Control(b) Curry sauce, (persons) (persons) Strength of meat flavor  8 7Preference of meat flavor 14 1 Strength of total flavor  7 8 Preferenceof total flavor 14 1 Preference of taste 15 0 Strength of taste 12 3Comprehensive evaluation 15 0 (0.05% by weight added) Inventive Control(c) Japanese consomme soup (persons) (persons) Strength of Japanesedashi stock flavor  9 6 Preference of Japanese dashi stock flavor 15 0Strength of total flavor  8 7 Preference of total flavor 14 1 Preferenceof taste 15 0 Strength of taste 15 0 Comprehensive evaluation 15 0

INDUSTRIAL APPLICABILITY

[0044] According to the present invention, a natural food materialcontaining cysteinylglycine at a high content such as a yeast extractcontaining cysteinylglycine at a high content, useful for improving foodflavors or the like, can be easily obtained.

[0045] Further, according to the present invention can be easilyobtained a food savor enhancer exhibiting no problems such as strongburnt smell or odor or chemical smell or odor and unpleasant pungentodor or unclear taste in the taste quality, and enhancing food savorsuch as meat flavor and the like suitable for foods and drinks.

1. A method for producing a food material containing cysteinylglycine ata high content, which comprises the step of (a) maintaining a startingfood material containing glutathione in a ratio of 1% by weight or morebased on the solid content at a temperature of 50 to 120° C. and a pH of1 to 7 in the presence of water, or (b) treating the food material witha γ-glutamylpeptide hydrolase at a temperature of 15 to 70° C. and a pHof 3 to 9 in the presence of water, whereby a food material rich incysteinylglycine is allowed to result.
 2. The method for producing afood material containing cysteinylglycine at a high content according toclaim 1, wherein said γ-glutamylpeptide hydrolase is a member selectedfrom the group consisting of γ-glutamyltransferase,γ-glutamylcyclotransferase, and glutaminase.
 3. The method for producinga food material containing cysteinylglycine at a high content accordingto claim 1 or 2, wherein said starting food material is a yeast extractor a yeast cell which is a natural food material.
 4. A method forproducing a food flavor enhancer, which comprises the steps of (a)adding a sugar to cysteinylglycine or a food material containingcysteinylglycine in a ratio of 0.5% by weight or more based on the solidcontent, and (b) heating the resulting mixture at a temperature of 70 to180° C. for a period of 10 to 180 minutes in the presence of water.